Magnetic and Electronic Properties of n-type (Al,Ga) co-doped Zn(Cu)O based Dilute Magnetic Semiconductors
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Magnetic and Electronic Properties of n-type (Al,Ga) co-doped Zn(Cu)O based Dilute Magnetic Semiconductors Deepayan Chakraborti1, John T. Prater1,2, and Jagdish Narayan1 1 Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27606 2 Materials Science Division, Army Research Office, Research Triangle Park, NC, 27709
ABSTRACT Systematic studies on the epitaxial growth and the effect of n-type (Al,Ga) doping on the magnetic and electrical properties of 2.0 % Cu doped ZnO dilute magnetic semiconducting thin films deposited on c-plane sapphire single crystals by pulsed laser deposition are reported here. An decrease in more than 3 orders of magnitude in resistivity from 2 x101 Ohm cm for the 2.0 % Cu doped ZnO to ~5 x10-3 Ohm cm for Al and Ga codoped films is observed. This increase in conductivity does not show any effect on ferromagnetic ordering thus contradicting the claim of free carrier mediated exchange as being responsible for ferromagnetic ordering in these DMS systems. A bound magnetic polaron or F-center mediated exchange is a possible explanation for the origin of ferromagnetism in these ZnO based DMS thin films.
INTRODUCTION The attractive property of incorporating both spin and charge functionality of the electron in a single material system has led to the search of Diluted Magnetic Semiconductors (DMS) displaying room temperature ferromagnetism (RTFM). Following a theoritical prediction by Dietl et al.[1] that heavily p type doped (3 x1020 cm-3) ZnO would display ferromagnetism, a large number of studies have been undertaken to dope ZnO with various transition metals to display RTFM. One of the earliest studies by Ueda et al. [2] showed RTFM in heavily n-type doped (3 x1020 cm-3) Co doped ZnO. Further works have also discovered ferromagnetism in ntype doped ZnO. The literature is abundant with contradictory reports about the magnetic properties of ZnO based DMSs.[3,4] There is even doubt about whether the ferromagnetism occurring is intrinsic to the material or arising due to formation of ferromagnetic secondary phases. The origin of ferromagnetism in ZnO based DMS materials systems is also unclear although the more conventional mechanism is the free carrier mediated exchange mechanism (RKKY)[2,5]. But this mechanism cannot explain the occurrence of ferromagnetism in materials with low carrier densities. Cu doped ZnO has attracted the attention of researchers since neither metallic Cu nor its oxides (CuO or Cu2O) are ferromagnetic, hence ruling out the possibility of ferromagnetism due to formation of clusters. We have reported in our previous [6] work the presence of RTFM in Cu:ZnO thin films that possess high magnetization(~0.75 µB/Cu atom) at low concentrations (1.0 -2.0 %) of Cu. Inspite of showing very good magnetic properties these films were very resistive (~ 0.5X106 Ohms at room temperature) which seemingly ruled out the possibility of free carrier mediated ferromagnetism in these films. Also the high resistance of these films might prove to be detrimental
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